FIGURE 1. Pedigree of family of the affected sisters. The serum concentrations of potassium and magnesium were normal in the individuals marked by asterisks.(With permission Trans. Assoc. Am. Phys.') been listed in the right-hand column. The lower limit of normal for serum magnesium in our laboratory is 1.45 mEq/l for individuals on a normal diet, and it is of interest that normal subjects respond to this low intake of magnesium by developing minimal but definite hypomagnesemia. On the sixth day of study, the urinary excretion of magnesium in normal subjects is trivial. In contrast, dl of the affected patients demonstrate a marked impairment in renal conservation. In patient AT, the study was terminated on the fifth day when her concentration of magnesium in serum fell to 0.32 milliequivalent/liter (mEq/l) with a concentration of magnesium in an ultrafiltrate of plasma of 0.20 mEq/l. Despite this striking hypomagnesemia, the patient felt well, the physical examination was unchanged, and the Chvostek and Trousseau signs were not present. Since rats with intact parathyroid glands develop hypercalcemia in response to magnesium depletion, u it was of interest to ascertain whether these patients would develop alterations in calcium metabolism in association with magnesium restriction. During the course of our experimental studies, hypercalcemia occurred in all three patients. It should be emphasized, however, that magnesium restriction was not invariably associated with this response. The influence of magnesium restriction on serum calcium and magnesium concentrations is displayed in FIGURE 4 in terms of a regression equation relating these two parameters. Patient GM invariably responded to magnesium withdrawal by developing hypercalcemia. It should be noted that a subtotal thyroidectomy had been performed on this individual several years previously for hyperthyroidism. Postoperatively,